On 02/02/2010 14:12, Daniel Bünzli wrote:
> The second one uses Maps for logarithmic time lookups. Operations on
> dictionaries are thread safe. However key creation is not because
> unique ids need to be generated for them.

FWIW, a thread-safe way to generate fresh ids is:

let fresh_id () = Oo.id (object end)


Also, the "perfect" solution you are referring to becomes in the syntax 
of OCaml's trunk:

let embed () (type s) =
   let module M = struct exception E of s end in
   (fun x -> M.E x), (function M.E x -> Some x | _ -> None)


-- Alain